Disallow adding ContextLifecycleObservers during iteration.

Source/core/dom/ContextLifecycleObserver.h

Index: Source/core/dom/ContextLifecycleObserver.h
diff --git a/Source/core/dom/ContextLifecycleObserver.h b/Source/core/dom/ContextLifecycleObserver.h
index e9b4c18e1248cd247cf564bbe4896b903fb0fa72..939e9be4614eef0cfc304c1071482caf03e6590a 100644
--- a/Source/core/dom/ContextLifecycleObserver.h
+++ b/Source/core/dom/ContextLifecycleObserver.h
@@ -47,7 +47,11 @@ public:
     Type observerType() const { return m_observerType; }
 
 protected:
-    explicit ContextLifecycleObserver(ExecutionContext*, Type = GenericType);
+    explicit ContextLifecycleObserver(ExecutionContext* executionContext, Type type = GenericType)
+        : LifecycleObserver(executionContext)
+        , m_observerType(type)
+    {
+    }
 
 private:
     Type m_observerType;